A fundamental feature of nervous systems is that they provide plasticity of structure and function which allows animals to adapt to changes in their environment. We are only beginning to understand the underlying mechanisms, the limitations, and the behavioral consequences, such as learning, of naturally occurring plasticity in the brain. The song control system in the avian brain provides excellent opportunities for addressing such basic issues of behavioral neuroscience. Seasonal changes of environmental factors, such as photoperiod, have a profound effect on birds and most other animals. In songbirds seasonal changes in photoperiod elicit changes in circulating concentrations of gonadal steroid hormones, which in turn cause changes in song behavior and in morphological and physiological attributes of the neuroendocrine system that controls song. The exchange of social cues between individuals enhances the growth of the song system during the breeding season. The goal of this proposal is to use a comparative approach to explore the proximate mechanisms and behavioral functions of seasonal plasticity in the avian song control system. We will determine whether seasonal changes in neuronal recruitment to song nuclei are causally related to seasonal death of neurons, and whether hormones play a neuroprotective role. We will examine how the electrophysiological properties of neurons change seasonally, and whether seasonal growth of song circuits is activity dependent. Tropical birds show seasonal plasticity of their song systems and we will test whether this is mediated by steroid hormones. The results of the proposed studies will increase our understanding of steroid hormonal influences on the nervous system, how steroid hormones can be used in the treatment of neurodegenerative diseases, and the relationship between plasticity in the adult brain and learning.